Summary/Abstract An important cause of preterm labor is immune infiltration into the placenta. Although the placental immune response likely starts long before delivery, no technique currently available permits in vivo, real-time assessment of placental immune responses during pregnancy. As a result, clinicians have limited ability to detect placental inflammation or intervene so as to reduce the immune response and prevent preterm delivery. This R01 application proposes to develop a novel, noninvasive human placental immune imaging (PII) technique, which will be able to safely assess human placental inflammation in real time, responding to RFA-HD-18-003: Moving Beyond Standard Assessments: Applying Novel Tools to Assess Human Placental Structure and Function in Real Time. PII will be based on a diffusion magnetic resonance imaging (MRI) technique called diffusion basis spectrum imaging, which can noninvasively image and quantify brain inflammation in multiple sclerosis in both animal models and human patients. Development of PII must take into account the fact that the human placenta is quite different from animal placentas and is a very dynamically changing organ throughout pregnancy. The anatomic and potential pathological complexity and heterogeneity of the placenta create strong background noise interference not present in the brain, making it more challenging to identify and extract the signals specific to placental inflammation. To address this technical challenge, this proposal will develop PII specifically for human applications by making use of three distinct clinical cohorts: those at low risk of preterm birth (Aim 1), and those at high risk of preterm birth who either respond or fail to respond to treatment to prevent preterm birth (Aims 2 and 3). In addition to performing PII on these women at two (Aims 2 and 3) or three (Aim 1) time points in pregnancy, this proposal includes measurement of immune factors and long non-coding RNAs in maternal blood, and histological characterization of inflammatory cells in placenta samples obtained at delivery. Completion of these aims will 1) yield a fully developed PII system, 2) refine PII through histopathological assessments of placentas and measures of systemic immune responses, 3) define the placental immune responses characteristic of healthy, term pregnancy, 4) identify placental immune responses characteristic of pregnancies at risk of preterm birth, and 5) begin to reveal immune signatures associated with success and failure of progesterone treatment. Because PII will require MRI sequences that are already approved by the Food and Drug Administration, its safety and great accessibility will allow it to be rapidly extended to clinical trials or clinical populations.